This invention relates to an inexpensive hydraulic brake system for vehicles that permit antilock brake control and vehicle stability control.
A vehicle hydraulic brake system with which antilock brake control (ABS) and vehicle stability control (VSC) are possible includes a hydraulic pressure source having a power pump for generating a predetermined hydraulic pressure, a master cylinder for generating hydraulic pressure corresponding to a force applied to the brake by a driver and/or the operation of automatic pressurizing means, wheel cylinders actuated by the hydraulic pressure applied from the master cylinder for applying braking force to the vehicle wheels, and wheel cylinder pressure control valves disposed in hydraulic lines that connect the master cylinder to the wheel cylinders, for adjusting pressure in the wheel cylinders.
Also known is a brake system to which is further added a pressure adjusting valve for adjusting the hydraulic pressure supplied from the hydraulic pressure source to a value corresponding to the brake operation and/or actuation of the automatic pressurizing means.
These hydraulic brake devices include a controller (that is, electronic control unit) that judges the necessity of wheel cylinder pressure adjustment based on the information from various sensors including wheel speed sensors, and if such adjustment is determined to be necessary, controls the wheel cylinder pressure control valves. For example, if it judges it necessary to reduce the pressure of the wheel cylinders, the controller will activate the wheel cylinder pressure control valves to close the hydraulic pressure supply lines to the wheel cylinders and open the discharge lines from the wheel cylinders.
The pressures in the wheel cylinders thus fall. Brake fluid discharged from the wheel cylinders is released into the atmospheric reservoir. That is, during every pressure reduction phase of such electronic brake control, i.e. computer-controlled brake operation, brake fluid is discharged into the reservoir, so that the piston of the master cylinder (hereinafter simply xe2x80x9cmaster pistonxe2x80x9d) gradually advances until it abuts the end wall of the cylinder. Once the master piston abuts the cylinder end wall, it is impossible to supply brake pressure any more from the master cylinder to the wheel cylinder.
The hydraulic brake system disclosed in Japanese patent publication 59-130769 has a hydraulic pressure supply means for introducing the pressure fluid from the pressure adjusting valve into the hydraulic line connecting the master cylinder with the wheel cylinders, if part of fluid in the above hydraulic line is lost and the amount of the fluid in this line is determined to be insufficient.
The hydraulic brake system disclosed in this publication includes a solenoid valve for closing the line connecting the hydraulic pressure supply means to the master cylinder, and/or a switch (stroke sensor) for monitoring the stroke of the master piston.
Once hydraulic pressure is supplied from a pressure adjusting valve into the hydraulic line connecting the master cylinder with the wheel cylinders, the difference between the pressure in a pressure chamber which is applied to the master piston in such a direction as to advance the piston and the pressure in the pressure chamber in the master cylinder (hereinafter xe2x80x9cmaster pressure chamberxe2x80x9d) will disappear, so that no pressure acts on the piston to retract it. Since the pressure difference is gone, unless any means for checking the retraction of the master piston is provided, the master cylinder would be pushed back under the force of the return spring to a position where the master pressure chamber communicates with the master cylinder reservoir. If this happens, the pressure output of the master cylinder will disappear.
Thus, the above publication proposes to close the line leading to the master cylinder with a solenoid valve to prevent the hydraulic pressure supplied through the hydraulic pressure supply means from flowing into the master hydraulic pressure chamber. In another embodiment, this publication proposes to detect the position of the master piston to temporarily stop the supply of brake fluid from the pressure adjusting valve before the master piston returns to a position where the master pressure chamber re-communicates with the reservoir for the master cylinder.
The hydraulic brake system disclosed in the Japanese patent publication 59-130769 needs expensive elements, such as a solenoid valve for closing the line between the master cylinder and the hydraulic pressure supply means and/or a sensor for monitoring the stroke of the master piston.
Another conventional brake system is adapted to release brake fluid discharged from the wheel cylinders into a low-pressure reservoir, draw up brake fluid in the low-pressure reservoir by means of a power pump and return the thus sucked up brake fluid into a line between the master cylinder and a master cylinder pressure control valve. This type of brake system requires another power pump for returning brake fluid in addition to a power pump used in the hydraulic pressure source. This pushes up the cost of the entire system.
An object of this invention is to provide an inexpensive hydraulic brake system for vehicles which permits antilock brake control, vehicle stability control and other electronic brake control.
According to this invention, there is provided a vehicle hydraulic brake system comprising a hydraulic pressure source for generating a predetermined hydraulic pressure, a pressure adjusting valve for adjusting the hydraulic pressure supplied from the hydraulic pressure source to a value corresponding to a brake operation and/or an automatic brake control, a master cylinder for generating hydraulic pressure according to brake operation and/or automatic brake control, wheel cylinders activated by the output pressure from the master cylinder to apply braking force to wheels of the vehicle, wheel cylinder pressure control valves provided in a hydraulic line connecting the master cylinder to the wheel cylinders for adjusting the hydraulic pressure in the wheel cylinders, characterized in that there are provided means for estimating the amount of pressure increase and/or pressure reduction adjusted by the wheel cylinder pressure control valves, and a hydraulic pressure supply unit for supplying hydraulic pressure output of the pressure adjusting valve into a hydraulic line connecting the master cylinder to the wheel cylinder pressure control valve according to the estimated amount of pressure increase and/or pressure reduction.
Instead of the pressure output of the pressure adjusting valve, the pressure output of the hydraulic pressure source may be supplied into the hydraulic line. In this case, a pressure adjusting valve may or may not be provided.
Hydraulic pressure may be supplied into the hydraulic line while the master cylinder has still enough room for further increasing or decreasing pressure. But this will unnecessarily increase the number of times hydraulic pressure is supplied. Thus, hydraulic pressure is preferably supplied only if the piston of the master cylinder has advanced near to its limit.
Hydraulic pressure may be supplied into the hydraulic line so that the piston of the master cylinder will move to a point near its initial position or to a predetermined position.
The xe2x80x9cpoint near an initial positionxe2x80x9d refers to a point which is near the initial position but where the communication between the master pressure chamber and the reservoir for the master cylinder will not restart. The xe2x80x9cpredetermined positionxe2x80x9d refers to a predetermined point in the stroke of the piston.
The means for estimating the amount of pressure increase may be of a type which estimates the amount of pressure increase from the pressure increase time period during which pressure is increased, or from the pressure increase time and the differential pressure between the pressure output from the master cylinder (which may be estimated from any pressure associated with the master cylinder pressure output or the brake operating force) and the wheel cylinder pressure (which may be estimated from the amount of pressure increase or reduction, deceleration of the vehicle, or deceleration of the wheels). It may be in the form of a logic circuit provided in the controller.
The means for estimating the amount of pressure reduction may be of a type which estimates the amount of pressure reduction from the pressure reduction time period during which pressure is reduced, or from the wheel cylinder pressure and the pressure reduction time period. This means, too, may be a logic circuit in the controller.
The hydraulic pressure supply unit may be formed by combining a solenoid valve for opening and closing the hydraulic pressure supply line extending from the pressure adjusting valve or the hydraulic pressure source, with a logic circuit in the controller which opens the solenoid valve if the estimated amount of pressure increase and/or the estimated amount of pressure reduction exceeds a threshold to supply brake fluid into the above-described hydraulic line, calculates the amount of brake fluid supplied from the time period during which the solenoid valve is open, or the differential pressure between the pressure of the brake fluid supplied and the pressure output of the master cylinder and the time period during which the solenoid valve is open, and closes the solenoid valve when a predetermined amount of brake fluid has been supplied into the hydraulic line.
By estimating the amount of pressure increase and/or pressure reduction and calculating the amount of brake fluid supplied, it is possible to estimate the position of the master piston and control the amount of brake fluid supplied without using a stroke sensor. This pushes down the cost of the system.
According to the present invention, since the amount of pressure increase and/or pressure reduction of the wheel cylinders are estimated and the fluid pressure supplied from the pressure adjusting valve or the fluid pressure source is introduced into the fluid line between the master cylinder and the wheel cylinder pressure control valve, a solenoid valve for shutting off the master cylinder from the fluid pressure supply unit or a sensor for monitoring the stroke of the master piston is needed any more. This cuts down the cost of the entire brake system.
There may be a difference between the estimated amount of pressure increase and/or pressure reduction and the actual amount of pressure increase and/or pressure reduction or between the calculated amount of brake fluid supplied and the actual amount of brake fluid supplied. But once the brake is released, the master piston will return to its original position, so that communication between the master pressure chamber and the master cylinder reservoir is resumed and the entire system is reset. Thus, such a difference will have no undue influence on the next braking operation when the brake pedal is depressed again.